In a device of this type which is disclosed in German Patent (OLS) No. 2,249,786, a gas generator is disposed in a piston and the piston is driven by means of a propellant generated by the gas generator. The piston is guided in a guide pipe and can be connected with a safety belt of a motor vehicle via a traction mechanism. A pressure chamber, from which the propellant generated in the interior of the piston is discharged via at least one outlet opening, is disposed in the guide pipe.